A lunite-Jarosite Group of Minerals. 213 



soda. The slight excess of Fe^Og and H^O, as indicated by the 

 ratio, is evidently due to some ferric hydroxide ; probably the 

 dark ferruginous impurities seen under the microscope are in 

 part responsible for this, and there are also traces of some 

 arsenate and silicate present. Regarding the excess of Fe203 

 and H^O as due to impurities, it is found that 94 per cent of 

 the material analyzed may be regarded as pure natrojarosite, 

 as indicated below : 



After deducting Theory for 

 impurities, Na2Fe6[OH]i2[S04]4. 



Fe^Og 46-43 or 49-39 49-49 



ISra.O--. 5-81 " 6-18 6-39 



K,0 0-35 " 0-37 



SO3 30-96 " 32-94 32-99 



HO 10-45 " 11-12 11-13 



94-00 100-00 100-00 



That six per cent of impurities should be present in a crys- 

 talline powder such as analyzed is not surprising, when it is 

 taken into consideration that it would require something like 

 2,500,000 crystals to make one gram of material, the estima- 

 tion being based on the assumption that the crystals are O-IO'""' 

 in axial diameter and 0-02™'° thick, which is certainly above 

 their average size. 



Among the specimens from Cook's Peak, ISTew Mexico, sent 

 to the Sheffield Laboratory by Mr. Porter, were some masses 

 of a rather firmly cemented aggregate of minute crystals of a 

 mineral of the jarosite group. The specimens are of a brown- 

 ish-yellow color, and have in places the glistening appearance 

 of a mica- schist. They also look as though they had been sub- 

 jected to pressure and had been somewhat sheared. The 

 material is rather easily crushed, and the powder when exam- 

 ined with the microscope exhibits the properties of the natro- 

 jarosite just described. The crystals are associated with a little 

 limonite and quartz, and pure material for analysis could not 

 be obtained. Only a partial analysis, therefore, was under- 

 taken with the following results : 



Fe^Og 55-60 



Na^O 4-49 



K,0 0-11 



PbO 0-96 



SO3 and HjO were present but not determined. The results 

 are sufiicient to indicate that the material is essentially natro- 

 jarosite. 



Plwmhojarosite. 



This material is from Cook's Peak, Kew Mexico. It occurs 

 as a glistening, crystalline powder and as loosely cohering 



